Loss of pwr steering on 2003 Jaguar XJ8

i lost pwr steering while driving. I checked pwr steering fluid it was good. So what should i do next to determine what wrong

Asked by for the 2003 Jaguar XJ8
You could have an issue with the servotronic system that controls the power assist to the steering system electronically to adjust sensativity to vehicle speed. Circuit testing at that valve must be done to determine if you have a pump issue or a servotronic valve issue.